Elastography techniques are devoted to measure biological tissues mechanical properties, as for example elasticity, in order to help medical diagnosis.
Usually, they are implemented in medical imaging systems as additional feature of an existing imaging modality such as MRI or ultrasound.
In this context, elastography gives new clinical information to the physician to help him in establishing a diagnosis.
Several elastography techniques have been developed. Some are currently in clinical evaluation and some are already embedded in a medical imaging product.
Schematically, elastography techniques can be divided into three different types: static, monochromatic or transient based techniques, depending on the characteristics of the mechanical excitation applied.
The present invention relates to transient elastography techniques that rely on the generation of a transient mechanical excitation in the body in order to deduce tissue mechanical properties.
Such methods can be classified according to the way this transient vibration is applied, externally, for example with a specific external device generating vibration, or internally, for example using the vibration generated by focalization of ultrasounds in a tissue resulting in an ultrasound radiation force.
Such methods can also be classified according to the imaging method, ultrasound or Magnetic Resonance Imaging for example.
All those elastography methods are imaging techniques in the sense that they define a continuous region of interest (ROI) in which imaging is performed in all this ROI and only in this ROI.
Indeed several local estimations of tissue mechanical properties are performed to give a viscoelastic map or elastogram in said predefined region of interest (ROI).
Usually those imaging techniques are time and processing consuming. Most of the time, they require huge amount of energy deposit in the tissue.
For those reasons they have not being implemented to date in real time on a medical imaging device.
Elastographic techniques are thus used punctually, for example once a lesion was located.
Nevertheless, in some cases, it is interesting to the physician to have a global and fast estimation of viscoelastic parameters of the ROI in an imaged tissue.
Such viscoelastic parameters enable to qualify the global mechanical behavior of the tissue.
Interesting applications concern pathologies inducing smooth spatial variations of the elasticity and are, for example, liver fibrosis evaluation, vascular diseases evaluation or muscles elasticity monitoring.
Such global information can also be very useful as a preliminary or calibration step to the imaging techniques cited above.
Today, only one ultrasound based technique proposing a global fast elasticity estimation of tissues is known from the document FR 2 791 136.
This technique is based on the concept of reducing the imaged region of interest to one ultrasound beam, imaging a shear wave propagation along the beam line and deducing a mean elasticity value along that line.
However such technique suffers from a major drawback.
It relies on the hypothesis that the elasticity value estimated along the single ultrasound line is a good and robust representation of the mean elasticity of the whole tissue.
This is usually not the case and such assumption leads to low performance regarding the statistical variance and the reproducibility of the measures.
It is thus challenging, with this technique, to distinguish early stage liver cirrhosis.
As a consequence there is a need for an Elastography technique able to measure a mean elasticity value of a given tissue without imaging the whole tissue and without making any assumption on the viscoelastic homogeneity of the tissue.